Bending Stresses:

(a) The maximum bending stresses in tension (σ_bt, _cal) or in compression (σ_bc, _cal) in extreme fibre calculated on the effective section of a beam shall not exceed the maximum permissible bending stress in tension (σ_bt) or in compression (σ_bc) obtained as follows:

σ_bt or  σ_bc = 0.66  f _y                                                                                                                    

(b) For an I-beam or channel section with equal flanges bent about the axis of maximum strength (x-x axis), the maximum bending compressive stress on the extreme fibre shall not exceed the values of maximum permissible bending compressive stress σ_bc given directly in Table 1(A and B) for steels with yield stress f_y = 250 MPa. [Table for other steels, e.g. f_g = 340 MPa and 400 MPa are given in IS: 800.]

(c) To built-up beams and plate girders a value of σ_bc is calculated by the following formula

σ_bc = 0.66 (f_cb. F_y/(fⁿ_cb +fⁿ_y)^1/n                                                                                 

where, f_cb = Elastic critical stress compute using Eq. (5) in MPa,

f_y = Yield stress of steel in MPa, and

n = A factor assumed as 1.4.

A values of σ_bc for various values of f_cb and f_y calculated as above can also be obtained from Table 1 (IS: 800).

(d) Values of the elastic critical stress fcb as obtained from Eq. (5) shall be increased by 20% when T/t ? 2.0 , and d₁/t ? 1344/√ f_y .

(e) Beams Bent about the Axis of Minimum Strength (y-y axis) : The maximum permissible bending in tension (σ _bt) or in compression (σ_bc) shall not exceed 0.66 fy.

(f) Angles and Tees : The bending stress in the leg when loaded with the flange or table in tension shall not exceed 0.66 f_y. When loaded with the leg in compression, the permissible bending stress shall be calculated according to above with k₂ = - 1.0 and T = Thickness of leg.